1. Field of the Invention
The present invention relates to a novel solid electrolyte gas-sensing device. More specifically, it relates to a solid electrolyte gas-sensing device for detecting inorganic gases such as carbon dioxide, nitrogen oxides and sulfur oxides in ambient atmosphere, which exhibits high sensitivity for gases to be measured, shows almost no variability from one device to another concerning the initial sensitivity and the sensitivity with time, and therefore has remarkably high reliability.
2. Description of the Related Art
Inorganic gases such as carbon dioxide, nitrogen oxides and sulfur oxides have been hitherto measured in various fields for utilizing the measurement data for fuel control or environmental measurement. For the measurement of these inorganic gases, there have been employed a variety of analysis methods such as an infrared absorption method, an ultraviolet absorption method and a chemiluminescence emission method, while it has been so far pointed out that these methods have a problem in that the apparatuses used therefor are large-scaled and expensive and require maintenance. On the other hand, a solid electrolyte gas-sensing device using a change in the electromotive force of a solid electrolyte layer has been developed as a small-sized, facile and less expensive gas-sensing device which can be a substitute for the above apparatuses.
In detecting inorganic gases such as carbon dioxide, nitrogen oxides and sulfur oxides by means of the above solid electrolyte gas-sensing device, mobile ion species of the solid electrolyte layer and the ion species generated from a gas to be measured are different in some cases. It is hence proposed to use a structure in which an auxiliary electrode comprising metal salt which shows dissociation equilibrium with a gas to be measured is provided on the working electrode side. For example, a solid electrolyte gas-sensing device is known in which the solid electrolyte layer is formed of .beta.-Al.sub.2 O.sub.3 or Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12 (wherein x is 0 to 3, generally referred to as "NASICON") as a sodium ion conductor and the auxiliary electrode is formed of a salt of the same metal as that which forms mobile ion of the solid electrolyte layer (Solid State Ionics 23 107-112, 1987, North-Holland, Amsterdam).
When the above-constituted gas-sensing device is placed in ambient atmosphere containing a gas to be measured, an electromotive force occurs between the two electrodes according to the following Nernst's equation, EQU EMF=C-RT/nF.multidot.1n [G] (1)
wherein EMF is an electromotive force of the solid electrolyte gas-sensing device, C is a constant, R is a gas constant, T is a temperature of the gas-sensing device, n is an order of reaction, F is a Faraday's constant and [G] is the concentration of a gas to be measured in ambient atmosphere.
The concentration of a gas to be measured can be determined on the basis of the above electromotive force.
FIG. 4 shows the structure of the above solid electrolyte gas-sensing device, in which a reference electrode layer 7 is formed on one surface of a solid electrolyte layer 9, and the other surface is provided with a working electrode layer laminate-formed of an electrode layer 14 and a metal salt as an auxiliary electrode 13. When a gas to be measured reaches the sensing device, a dissociation equilibrium reaction takes place only in the working electrode layer formed of the metal salt 13 and the electrode 14 to cause a change in metal ion amount. This change in the metal ion amount is measured as a change in the electromotive force between the two electrodes of a battery of the solid electrolyte layer 9 according to the above equation (1) with a voltmeter 6, whereby the sensing device can measure the concentration of a gas to be measured. In FIG. 4, numeral 1 indicates a power source, numeral 2 indicates a heater, numeral 3 indicates an alumina substrate, numeral 4 indicates an adhesive, and numeral 5 indicates a lead wire.
However, the solid electrolyte gas-sensing device provided with the above auxiliary electrode has a problem in that the initial sensitivity and the sensitivity with time greatly vary from one device to another and that it is difficult to control the product quality in industrial production. This problem is particularly serious when the mobile ion of the solid electrolyte layer differs from the metal ion of the metal salt as the auxiliary electrode.
Further, as shown in FIG. 5, there is known a carbon dioxide-sensing device having a structure in which an auxiliary electrode 15 formed of a metal salt that shows dissociation equilibrium with a gas to be measured is formed on a solid electrolyte layer 9 and an electrode layer 16 is further laminated thereon.
Japanese Laid-open Patent Publication No. 195,246/1990 discloses a carbon dioxide gas sensor comprising an ion-conducting ceramic plate formed of a solid electrolyte having two electrode layers, one electrode layer on one surface and the other on the other surface, a gas-sensing portion formed of a metal carbonate which is dispersed in one of the two electrode layers and forms dissociation equilibrium with carbon dioxide gas and two lead wires, one wire bonding to one electrode and the other wire bonding to the other electrode, and a heating portion for heating the above gas-sensing portion to a working temperature. Example of the above Japanese Laid-open Patent Publication discloses a carbon dioxide gas sensor having a cathode layer produced by uniformly coating an NASICON plate surface with a dispersion containing an Au paste as an electrode material and 10 to 50 wt %, based on the Au paste, of sodium carbonate, drying the coating and then sintering.
Japanese Laid-open Patent Publication No. 213,565/1992 discloses a CO.sub.2 gas sensor comprising a CO.sub.2 detecting portion formed of a solid electrolyte, a CO.sub.2 -sensitive electrode formed on the solid electrolyte and a CO.sub.2 -insensitive electrode formed on the solid electrolyte and a self-heating heater, the CO.sub.2 -sensitive electrode being formed from Au metal and other component and having pores through which a gas can at least diffuse. The above Japanese Laid-open Patent Publication discloses that the CO.sub.2 -sensitive electrode is formed from an Au paste and 30% by weight or less of a solid electrolyte.
The above-structured solid electrolyte gas-sensing device is still to be improved in sensitivity.